Brother MFC620CN Service Manual page 107

What's so bad about air bubble growth?
Pressure fluctuations do not affect the volume of a liquid (ink), but Boyle's Law says that the volume
of a gas (air) varies with the pressure on it. The main issue with air bubbles in the ink flow path is
their damping effect, absorbing the pressure from the piezoelectric ceramic actuators and thus
degrading ink-jet performance.
The above illustration shows an example of an extremely large air bubble reaching the print nozzle
from the channel ink flow path; the dotted line, the changes resulting from applying a voltage to the
laminated electrode, expanding the piezoelectric ceramic actuator. Note how contraction of the air
bubble absorbs the pressure from the piezoelectric ceramic actuator, preventing ink ejection.
Bubble (Without pressure)
Pressure chamber
(Channel)
3-19
Piezoelectric element
Bubble (High pressure)
Confidential